In recent years in Japan, nearly all of the secondary batteries installed in portable electronic devices, such as cellular phones and notebook personal computers, are lithium secondary batteries. In addition, lithium secondary batteries are also expected to be used practically in the future as large-scale batteries of hybrid electric vehicles, electrical power load-leveling systems, and the like, and their importance is continuing to increase.
These lithium secondary batteries have as major constituents thereof, a positive electrode and a negative electrode, each of which contains a material capable of reversibly inserting and extracting lithium ions, and further have a separator containing a non-aqueous electrolyte or solid electrolyte.
Among these constituents, examples of materials that have been examined for use as electrode active materials include: oxides, such as lithium cobalt oxide (LiCoO2), lithium manganese oxide (LiMn2O4), and lithium titanium oxide (Li4Ti5O12); metals, such as metal lithium, lithium alloy, and tin alloy; and carbon-based materials, such as graphite, and mesocarbon microbeads (MCMB).
Although the battery voltage is determined according to the difference in chemical potential between these materials at their lithium contents in the respective active materials, there is a possibility to obtain a large potential difference by a specific combination of these materials, and this is one of the characteristics of lithium secondary batteries excellent in energy density.
In particular, a combination of a lithium cobalt oxide (LiCoO2) active material and a carbon material which are used for electrodes enables a voltage of nearly 4 V, while the combination realizes a large charge/discharge capacity, which corresponds to the amount of lithium able to be extracted from and inserted into an electrode, and the combination also results in a higher degree of safety. Therefore, the combination of these electrode materials is widely used in current lithium secondary batteries.
On the other hand, when combining electrodes containing a spinel-type lithium manganese oxide (LiMn2O4) active material and a spinel-type lithium titanium oxide (Li4Ti5O12) active material, the lithium insertion-extraction reaction tends to occur smoothly and there are fewer changes in crystal lattice volume accompanying the reaction. Therefore, it has been identified that the resultant lithium secondary battery can be excellent in long-term charge/discharge cycle properties, and such a lithium secondary battery is developed for practical use.
Since chemical batteries, such as lithium secondary batteries and capacitors, are expected to be required to have larger size and longer life for use as electric vehicle power supplies, large-capacity backup power supplies, and emergency power supplies, electrode active materials offering even higher performance (higher capacity) have come to be required by combining oxide active materials like those described above.
Among these, since titanium oxide-based active materials exhibits a voltage of about 1 to 2 V in the case of using metal lithium for the counter electrode, studies have been conducted on materials having various crystal structures regarding their potential for use as electrode active materials in negative electrode materials.
In particular, titanium dioxide active materials having a sodium bronze-type crystal structure (herein, “titanium dioxides having a sodium bronze type-crystal structure” are abbreviated as “TiO2 (B)”), which can achieve a smooth lithium insertion/extraction reaction comparable to that of spinel-type lithium titanium oxide, while realizing a higher capacity than spinel types, are attracting attention as electrode materials (see Non-Patent Literature 1).
Among these, a method of producing TiO2 (B) has been clearly determined in which H2Ti3O7 is used as a starting material, and has been clearly demonstrated to enable the synthesis of an electrode material having TiO2 (B) as a main component thereof by heating in the air at a temperature of 400° C. or higher (see Patent Literature 1).
However, when TiO2(B) is used in an electrode, the irreversible capacity of the initial cycle is large, and there is a problem with the use of TiO2(B) as a negative electrode material in high capacity lithium secondary batteries.
On the other hand, it is clearly shown that H2Ti12O25 is generated by a heat treatment in a lower temperature region, such as 150° C. to 280° C., in the production process which uses H2Ti3O7 as a starting material (Patent Literature 2).
When this H2Ti12O25 is used in an electrode, the irreversible capacity in the initial cycle is small, and a high capacity of greater than 200 mAh/g can be attained. Therefore, the H2Ti12O25 is expected to be useful as a high capacity oxide negative electrode material.
However, in a production method using the H2Ti3O7 as a starting material, a plurality of quasi-stable phases occur as a result of a heat treatment at from 150° C. to 500° C. Therefore, for the production of H2Ti12O25 and TiO2(B), precise temperature control is required, and it poses a problem to adopt the production method as a production process in an industrial scale (see Patent Literature 2, and Non-Patent Literature 2).
It is known that the crystal structures of these titanium oxides have similar tunnel structures. Among them, H2Ti12O25 has a Na2Ti12O25 type tunnel structure, and as shown in FIG. 1, H2Ti12O25 is characterized by having a crystal structure having two types of tunnel spaces with different sizes, due to the skeletal structure established by TiO6 octahedrons connected to one another (see Patent Literature 2).
Furthermore, as shown in FIG. 2, TiO2(B) is characterized by having a crystal structure having one type of a small tunnel space, due to the skeletal structure established by TiO6 octahedrons connected to one another (see Non-Patent Literature 1).
The crystal structures of the two compounds are very similar to each other, and since the partial structure of H2Ti12O25 coincides with the crystal structure of TiO2(B), the presence of an intergrowth phase having a crystal structure which is intermediate of the two compounds as shown in FIG. 3, has been predicted from a crystallographic viewpoint.
The intergrowth phase as used herein refers to the case in which the X-ray diffraction diagram of the phase yields a pattern that is similar to the diagrams of both H2Ti12O25 and TiO2(B), but the diffraction peak positions do not match with the diffraction peak positions of none of the two compounds, and the phase has a crystal structure which is intermediate of the two compounds, as a mean structure. Thus, the intergrowth phase is not a simple mixture. Actual examples of the intergrowth phase are reported in, for example, titanium dioxide (see Non-Patent Literature 3).
However, in the known production method as described above, which uses H2Ti3O7 as a starting material, the presence of the intergrowth phase was not confirmed, and the X-ray diffraction diagram which would serve as a ground indicating the presence of the intergrowth phase, is not known.